The present invention generally relates to light emitting diode arrays. More particularly, the present invention relates to a light emitting diode (LED) lamp assembly manufactured in such a manner so as to have an integrated heat sink to avoid degrading temperatures otherwise created by the light emitting diodes.
Light emitting diodes (LEDs) have been available since the early 1960s in various forms, and are now widely applied in a variety of ways, including signs and message boards, vehicle lights, and even interior lights. The relatively high efficiency of LEDs is the primary reason for their popularity. Tremendous power savings are possible when LEDs are used to replace traditional incandescent lamps of similar luminous output.
One aspect of high powered LED technology that has not been satisfactorily resolved is the removal of heat generated by the LED. LED lamps exhibit a substantial light output sensitivity to temperature, and in fact are permanently degraded by excessive temperature. Recently, very high performance LEDs have been commercialized. However, these LEDs emit a substantial amount of heat during their operation, which can permanently degrade the LED over time, resulting in a lower output and operating life. In ideal conditions, the life of the LED is 50,000-100,000 hours, however, this life can be shortened to less than 10% of the designed life due to the heat generated by these new super bright LEDs. Until recently, the higher light output was the trade-off for the shortened life due to the heat it generated.
To maximize the life of LEDs a heat sink coupled to the LEDS has been increasingly used. For example, aluminum or metal core printed circuit boards (PCB) have been used. These PCBs have a dielectric layer on top of the metal surface which acts as an electrical insulator between the circuitry and the metal base. The circuit traces are then formed on top of the dielectric, and the electronic components attached thereto. There are several ways to manufacture a metal core PCB. For example, thin FR4 or fiberglass circuit board that already has the circuitry printed onto it is mounted onto a metal substrate. Another method is to print the circuitry onto the dielectric material after it has already been mounted onto the metal substrate. U.S. Pat. No. 4,729,076 to Masamai et al., discloses an LED lamp assembly wherein a heat absorber, in the form of an electrically insulating sheet, is disposed between the circuit board holding the LEDs and the heat sink.
However, these arrangements have several disadvantages. These PCBs still have thermal conductivity problems when used with high output LEDs, and do not perform well due to core thermal conductivity of the dielectric. When the LED is attached to a standard PCB, the heat is trapped under the LED due to the low thermal conductivity of the fiberglass circuit board, insulating sheet, etc.
U.S. Pat. No. 5,857,767 discloses an LED assembly wherein an insulating coating is disposed over an aluminum heat sink. Circuit traces are printed on the insulating layer and the leads of the LEDs are adhesively secured to the circuit traces to hold the LEDs in position. As disclosed by the '767 patent, the electrically conductive adhesive comprises an organic polymeric material compounded with a metal.
However, the adhesive attachment of the heat sink LEDs to the metallic heat sink in this manner, while an improvement of the other methods, still presents some disadvantages. The organic conductive adhesive does not conduct heat as well as metal. Thus, once again, heat can be trapped under the LED. LEDs typically include a heat sink slug in a central portion thereof immediately below the light emitting chip. Other components surrounding the light emitting chip are typically plastic or silicon and not intended to transmit the heat away from the chip. Attaching the bottom of the LED to the circuit traces with an adhesive serves to trap the heat within the LED.
Another disadvantage is that adhesive attachment systems sometimes exhibit detachment due to repeated temperature cycles. The relatively large differences in thermal expansion coefficients of plastics, epoxies and metals can give rise to substantial stresses of the attachment region. These stresses may exceed the strength of the adhesive and may eventually result in joint failure. This could result in the LED being electrically disconnected from the circuit.
Accordingly, there is a continuing need for a method of manufacturing LED assemblies in order to achieve the desired heat rejection from the LED. In order to utilize the inherent low temperature resistance that is available in the newer generation of high performance LEDs, special attention must be given to the attachment of these LEDs to the heat dissipater. The present invention fulfills these needs and provides other related advantages.